1. Field of the Invention
The present invention relates to a Coriolis mass flowmeter that generates a Coriolis force, i.e. a force that acts in proportion to the mass flow rate of a fluid flowing through a pipe by forcibly oscillating the pipe and detecting the phase difference of the oscillations on the upstream side and the downstream side of the pipe to determine a mass flow rate.
2. Description of the Related Art
A Coriolis flowmeter forcibly oscillates with an oscillator a fluid pipe that passes a fluid as an object of measurement, and detects the phase difference of the oscillations that is generated on the upstream side and the downstream side of the flow path in accordance with the flow rate of the mass of the fluid, thereby to determine a mass flow rate on the basis of the phase difference.
In order to generate an oscillation with a good efficiency, it is general to oscillate the Coriolis flow meter at a characteristic frequency of the fluid pipe as a beam. Major elements that determine this characteristic frequency of the fluid pipe are the elastic modulus (longitudinal elastic coefficient (Young's modulus)) f the fluid pipe material and the shape of the fluid pipe. A pipe made of metal is often used as a fluid pipe of a mass flowmeter because this elastic modulus is stable in terms of the temperature and the shape is stable with lapse of time.
On the other hand, when a fluid that corrodes a metal is used as an object of measurement, or when one wishes to prevent elution of metal ions to the measurement fluid, it is necessary to perform chemical resistance treatment on the inner side surface of the fluid pipe made of the metal. As an example thereof, Japanese Utility Model Application Laid-Open (JP-A) No. S64-15921 (front page) discloses a flowmeter in which the inner circumferential surface of the fluid pipe is coated with fluororesin. According to the invention of JP-A No. S64-15921, when the pipe diameter is small, the coating is difficult, and in particular, when there is a bent portion, a degree of difficulty is increased.
Also, as another requirement, there is a requirement that no metal is used at all as a material of the pipe because a fluororesin coating has a difficulty in increasing the film thickness and also it is difficult to confirm whether the coating has been done with certainty, whereby the fear of elution of the metal ions into the fluid pipe cannot be eliminated. Therefore, a fluid pipe made of a fluororesin alone (PFA tube) is proposed, for example, in WO 97/44640.
However, when the fluid pipe is made of fluororesin alone, there is a fear of change in the elastic modulus by temperature or creep phenomenon with lapse of time. In principle, the Coriolis mass flowmeter performs measurements by oscillating the fluid pipe. Since the change in the elastic modulus affects the oscillation, it may hinder a correct measurement of the mass flow rate.
Further, in the case of a PFA tube, when the fluid flowing through the fluid path is a chemical drug, the chemical drug penetrates through the tube, whereby an ambient gas such as water vapor or ammonia reacts with the chemical drug. The reaction product thereof is precipitated on the outer surface of the tube, thereby decreasing the mechanical strength of the tube.